This invention relates to a printer in which a print head moves along a track, and a system to control the position of the print head. In particular, a light source and photodetector produce a control signal for driving a print head mount which adjusts the position of the print head.
In a printer in which a print head moves along a track across a page, the accuracy of the image printed on the page is dependent on how precisely the print head can be positioned. This is especially important in dot-matrix printers where an image is built from a mosaic of many dots, or in a color printer where individual colored dots are overlaid to blend their colors. It is known to use rigid materials to prevent undesired flex or sag of components, however this is expensive and cannot be rapidly adjusted as the printer is operating to compensate for acceleration, vibration, or other dynamic forces.
It is known to measure the horizontal, or X direction, movement of the print head along the track by mounting a detector on the head and moving it past a "picket fence" parallel to the track. By counting pulses from the detector, the position and speed of the head can be determined. Knowing the position and speed of the head, the precise time to activate the printing can be calculated. This allows precise positioning of print in the X direction along the track. However, no similarly accurate and affordable method of precise positioning of the print head was known for the other translational axes of movement: in the Y direction, up and down the page; or in the Z direction away from the surface of the page. It is also possible for a print head to twist or rotate around its X, Y, or Z axes, and no affordable method of measuring or controlling such twist was known.
What was desired, and is provided by this invention, is a control system in which the position of the print head can be both measured and adjusted in any of the x, y, or z translation or rotation directions. This allows very precise printing using a moving print head.